The Conventions apply to all cases of declared war

between signatory nations. This is the original sense of applicability, which

predates the 1949 version.

The Conventions apply to all cases of armed conflict

between two or more signatory nations, even in the absence of a declaration of

war. This language was added in 1949 to accommodate situations that have all

the characteristics of war without the existence of a formal declaration of

war, such as a police action (a military action undertaken without a formal

declaration of war). The Conventions apply to a signatory nation even if the

opposing nation is not a signatory, but only if the opposing nation “accepts

and applies the provisions” of the Conventions.